Study of the Pseudo-equilibrium During Osmotic Dehydration of Apples and Its Effect on the Estimation of Water and Sucrose Effective Diffusivity Coefficients

The study of the equilibrium is needed not only for modeling of the osmotic process as a unit operation but also for a better understanding of the mass transfer mechanisms involved in this kind of systems. A true equilibrium state usually takes very long time to achieve; therefore, a pseudo-equilibrium state is often employed. Experimental pseudo-equilibrium states for water loss and solid gain during the osmotic dehydration of apple slices (5 x 50 x 50 mm(3)) at different osmotic syrup concentrations (30%, 40%, 50%, and 60% (w/w) of sucrose) were evaluated. Four empirical mathematical methods (Slopes, Azuara, Zugarramurdi and Lupin, and Equal concentration) were used in order to calculate the pseudo-equilibrium values obtained and then to compare them against to the experimental ones. Additionally, the effective diffusion coefficients for water and sucrose were calculated by using those pseudo-equilibrium values. Experimental pseudo-equilibrium values increased with concentration of the osmotic syrup, ranged between 24% and 56% for water loss and 11% and 28% for solid gain; the predicted pseudo-equilibrium values followed the same trend. The decreasing order of accuracy for pseudo-equilibrium values and effective diffusion coefficients, among the methodologies evaluated, was Equal concentration method > Azuara method > Slopes method > Zugarramurdi and Lupin method. Although the Equal concentration method has no theoretical accuracy, it is independent of the kinetic data presenting a higher advantage over the other three mathematical methods evaluated.